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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer2"></a><a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer asynchronously</a>
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<p>
        This tutorial program demonstrates how to use asio's asynchronous callback
        functionality by modifying the program from tutorial Timer.1 to perform an
        asynchronous wait on the timer.
      </p>
<pre class="programlisting">#include &lt;iostream&gt;
#include &lt;asio.hpp&gt;
</pre>
<p>
        Using asio's asynchronous functionality means having a callback function
        that will be called when an asynchronous operation completes. In this program
        we define a function called <code class="computeroutput">print</code> to be called when the asynchronous
        wait finishes.
      </p>
<pre class="programlisting">void print(const asio::error_code&amp; /*e*/)
{
  std::cout &lt;&lt; "Hello, world!" &lt;&lt; std::endl;
}

int main()
{
  asio::io_context io;

  asio::steady_timer t(io, asio::chrono::seconds(5));
</pre>
<p>
        Next, instead of doing a blocking wait as in tutorial Timer.1, we call the
        <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
        function to perform an asynchronous wait. When calling this function we pass
        the <code class="computeroutput">print</code> callback handler that was defined above.
      </p>
<pre class="programlisting">  t.async_wait(&amp;print);
</pre>
<p>
        Finally, we must call the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        member function on the io_context object.
      </p>
<p>
        The asio library provides a guarantee that callback handlers will only be
        called from threads that are currently calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        Therefore unless the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        function is called the callback for the asynchronous wait completion will
        never be invoked.
      </p>
<p>
        The <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
        function will also continue to run while there is still "work"
        to do. In this example, the work is the asynchronous wait on the timer, so
        the call will not return until the timer has expired and the callback has
        completed.
      </p>
<p>
        It is important to remember to give the io_context some work to do before
        calling <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>.
        For example, if we had omitted the above call to <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>,
        the io_context would not have had any work to do, and consequently <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a> would have
        returned immediately.
      </p>
<pre class="programlisting">  io.run();

  return 0;
}
</pre>
<p>
        See the <a class="link" href="tuttimer2/src.html" title="Source listing for Timer.2">full source listing</a>
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        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
      </p>
<p>
        Previous: <a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a timer
        synchronously</a>
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<p>
        Next: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments
        to a handler</a>
      </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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